The muscarinic acetylcholine receptor (mAChR) family consists of five GPCRs that mediate some of the neurotransmission functions of acetylcholine in the CNS and the periphery. The M1, M3 and M5 receptors couple to Gq to mobilize intracellular calcium, whereas the M2 and M4 receptors couple to Gi/o to inhibit cAMP production (Caulfield and Birdsall, 1998). M3 is expressed prominently in smooth muscle, and plays a primary role in mediating mAChR agonist-induced contractility. Mice lacking M3 have dilated pupils, which indicates a role for M3 in regulating tone of the pupillary sphincter muscle. In addition, M3 plays a role in feeding, as indicated by the lean and hypophagic phenotype of M3-null mice (Wess, 2004). Chemicon's cloned human M3-expressing cell line is made in the Chem-1 host, which supports high levels of recombinant M3 expression on the cell surface and contains high levels of the promiscuous G protein Gα15 to couple the receptor to the calcium signaling pathway. Thus, the cell line is an ideal tool for screening for antagonists of interactions between M3 and its ligands.